Over the last several years, capacitive touch pad pointing devices have entered widespread use in personal computers. There are at least three distinct capacitive sensing technologies used in touch pad devices today:                1. The “Field Distortion” approach, used by Cirque and Alps as described in PCT Application No. US90/04584, Publication No. WO91/03039 to Gerpheide. Specifically, Gerpheide teaches the application of an oscillating potential of a given frequency and phase to all electrodes on one side of a virtual dipole, and an oscillating potential of the same frequency and opposite phase to those on the other side. Electronic circuits develop a “balanced signal” which is zero when no finger is present, and which has the polarity of a finger on one side of the center of the virtual dipole, and the opposite polarity of the finger on the opposite side. To characterize the position of the finger initially, the virtual dipole is scanned sequentially across the tablet. Once the finger is located, it is “tracked” by moving the virtual dipole toward the finger once the finger has moved more than a row or column of the matrix constituting the capacitive sensor touch pad. Because the virtual dipole method operates by generating a balance signal that is zero when the capacitance does not vary with distance, it only senses the perimeter of the finger contact area, rather than the entire contact area.        2. The charge-detection approach used by the present assignee described in its U.S. Pat. No. 5,374,787 to Miller et al. Specifically, the present assignee employs what is called a “finger pointer” technique. This approach is to provide a position sensing system including a position sensing transducer comprising a touch-sensitive surface disposed on a substrate, such as a printed circuit board, including a matrix of conductive lines. A first set of conductive lines runs in a first direction and is insulated from the a second set of conductive lines running in a second direction generally perpendicular to the first direction. An insulating layer is disposed over the first and second sets of conductive lines. The insulative layer is thin enough to promote significant capacitive coupling between a finger placed on its surface and the first and second sets of conductive lines. Sensing electrodes respond to the proximity of a finger to translate the capacitance changes of the conductors caused by the finger proximity into position and touch pressure information.        3. An unrelated approach employed currently by Logitech.        
All three of these technologies share an important common feature: The finger is detected by a plurality of horizontally-aligned sensor electrodes disposed on a first layer, separated by an insulator from a plurality of vertically-aligned sensor electrodes disposed on a second layer. Such sensor electrodes are often formed as, but are not limited to, standard copper printed circuit board traces.
An example of such an electrode arrangement is shown in FIG. 1. Specifically, reference is made to FIGS. 1A through D, top, bottom, composite and cross-sectional views, respectively. Sensor array 10 is provided comprising substrate 12 including a set of first conductive traces 14 disposed on top of surface 16 thereof and run in a first direction to comprise row positions of sensor array 10. The set of second conductive traces 18 are disposed on a bottom surface 20 thereof and run in a second direction preferably orthogonal to the first direction to form the column positions of the sensor array 10. The set of first and second conductive traces 14 and 18 are alternately in contact with periodic sense pads 22 comprising enlarged areas, shown as diamonds in FIGS. 1A-1C. While sense pads 22 are shown as diamonds in FIGS. 1A-1C, any shapes such as circles, which allows close packing the sense pads 22 is equivalent for purposes of this discussion.
It is well recognized that capacitive touch pads, such as those described above, work well with fingers, but are normally unable to sense a pen or stylus. Capacitive touch pads are typically used as pointing devices. Resistive touch pads work well with pens, but require an uncomfortable amount of pressure when used with fingers. Resistive touch pads are typically used as writing or drawing input devices. To date, there has not been a practical touch pad which would work well with both fingers and pens along with a single input device to serve both functions. Such a touch pad would be especially valuable in portable applications where space is at a premium.
It is thus an object of the present invention to provide an input device in the form of a touch pad module which will accept both finger and stylus input, that is, having the desirable attributes of both a capacitive touch pad for finer input and a resistive touch pad for stylus input in the same module.
This and further objects will be more readily apparent when considering the following disclosure and appended claims.